The present invention relates to a knock control system of an industrial engine, and more particularly, to an engine knock control system provided with a capacitive discharge type ignition device for detecting knocking in industrial engine and for controlling an ignition timing of the engine.
In a four-cycle engine with an ignition device using a battery, it is well known that in order to avoid knocking due to abnormal burning of fuel, the knocking in the engine is detected by means of a knock sensor and a signal representing the generation of the knocking is transmitted therefrom to the ignition device to thereby retard the ignition angle or the ignition timing.
On the other hand, in a two-cycle engine, an electrode of an ignition plug is contaminated by oil contained in air-fuel mixture, causing misfiring. Accordingly, there has been widely utilized a capacitive discharge type ignition device (CDI) having a large initial discharge energy.
In the CDI, as is well known, a capacitor connected to a primary side of an ignition coil is charged by a relatively large voltage, for example, of 400 V and is discharged by a switching element such as a thyristor with a predetermined timing to generate a secondary voltage having a large initial discharge energy on a secondary side of the ignition coil. When the battery is utilized as a power source, it is necessary to have a A.C.-D.C. converter, thus increasing the cost. Accordingly, despite this defect, the CDI in which a magneto requiring less cost is utilized as the power source has been used in practice.
However, the CDI including the magneto as the power source discharges the capacitor by turning the thyristor on in response to a signal from a coil provided for the magneto, so that ignition is carried out in accordance with a fixed ignition timing, thus making it difficult to easily change the ignition timing.
In order to obviate the above defect, the Japanese Patent Laid-open No. 56-92357 discloses technology in which a trigger signal of the thyristor is advanced or retarded in angles from a signal generated in the coil of the magneto by an ignition timing computing means including an integrator comprising a capacitor and an amplifier in response to output of a thermistor for detecting a temperature of an engine oil.
However, according to this prior art, in a case where the knock sensor is used in place of the thermistor and the output of the knock sensor is input into the ignition timing computing means through the integrator, the ignition timing of the CDI can be retarded when the knocking is detected, but the ignition timing immediately advances in angle to the original ignition timing when the knocking is extinguished.
During such an operation, the ignition timing is frequently advanced or retarded repeatedly, resulting in lowering the durability of the engine and causing engine torque change, thus making the engine unstable in operation.
In order to obviate these defects in the prior art, it will be required to utilize a complicated circuit arrangement, resulting in the cost increase even in the usage of the CDI.